1. Field of the Invention
The present invention relates to a micro electro mechanical system (MEMS) resonating accelerometer, and more particularly, to a MEMS resonating accelerometer having improved temperature sensitivity.
2. Description of the Related Art
With the advance of microfabrication technology employing microelectromechanical system (MEMS) techniques, the recently proposed accelerometers have been downsized, more sophisticated, and reduced in price. At early development stages of MEMS technology, the main stream was piezoresistive accelerometers using piezoresistive characteristics based on silicon microfabrication technology in semiconductor manufacturing processes. However, recently, capacitive accelerometers are continually dominating the piezoresistive accelerometer market, except for particular fields requiring high G detection.
One of various advantages achieved by MEMS technology in developing MEMS accelerometers is a downsized dimension. Examples of application fields of miniaturized and low-priced MEMS inertial sensors include a car navigation system, automotive air-bag control, avoidance of jiggling a camera or video, a mobile telephone, robot posture control, gesture input recognition for a game, and detection of rotation and impact on HDD, and displacement type accelerometers (e.g., a piezoresistive accelerometer, a capacitive accelerometer, etc., in which a displacement of inertial masses in an accelerometer is changed by the applied acceleration and the displacement is converted to a voltage to measure the applied acceleration), are generally used.
MEMS accelerometers, aimed at attainment of performance comparable to the conventional mechanical accelerometer in addition to the downsized dimension, are mostly applied to navigation systems to replace the conventional mechanical accelerometer. Resonant accelerometers are generally used as navigational accelerometers.
The resonant accelerometer includes an inertial mass part whose displacement is generated by an external acceleration, a spring part limiting the direction of the mass movement in one direction while supporting the inertial mass part, and a resonator part whose frequency changes due to a tensile or compressive force. When acceleration is externally applied, the inertial mass part moves and the resonator part connected to the inertial mass part receives the tensile or compressive force according to the direction of the externally applied acceleration. The resonator part having received the tensile or compressive force may have a decreasing or increasing resonance frequency. The external acceleration is calculated based on the changed resonance frequency.
According to the resonating accelerometer, the external acceleration is measured based on a change in the resonance frequency. Accordingly, it is necessary to minimize error factors, which may cause a change in the resonance frequency, since factors other than external acceleration can change the resonance frequency. Therfore the performance of the resonating accelerometer is determined by the error factors.
Examples of the error factors affecting the resonance frequency include a change in the material's Young's modulus, change in the stress due to material's thermal expansion, stress due to differences between thermal expansion coefficients of different materials used, noises/vibrations due to external environment factors, and more. Specifically, the change in the stress due to the material's thermal expansion is determined by the shape of a structure, and a numerical value indicating the change in the stress due to the material's thermal expansion varies with the structure shape. When a compressive force or a tensile force is applied to the resonator by thermal expansion, there is a change in the output of the accelerometer as if external acceleration is applied to the resonator. It is difficult to separate the output by external acceleration and thermal expansion. In a state where the external acceleration is not applied, there is a change in the accelerometer output, causing an error.